Drives (also known as “drive units”), which are used to control electric motors via an electrically conductive cable, often have their own power rating, which sets a maximum allowable power for the drive. The power rating of drives is set such that, under certain conditions, the temperature of an inverter in the drive will not exceed a predetermined value. This is to ensure that the operating conditions of the drive remain safe. The effect of using a power cable of significant length is that it has the characteristics of a transmission line. It is well known that a badly terminated transmission line will cause current reflections. The screened power cables used with drives have an inherent capacitance. When a current is output from the drive and passes through the cable, some of this current is used to charge the cable, rather than reaching the desired destination of the motor. In general, the reflected cable charging current seen by the components of the inverter within a drive and its effect on the power loss of these components varies with the length of the cable.
When a drive is connected to (e.g.) a motor using screened cables, the cable charging current is added to the output current during the switching event. As the length of the cable is increased, the time taken for the reflected charging current to reach the drive increases. The effect of the cable length on the switching loss is at a maximum when reflection occurs after the switching event has finished. At every switching edge, the capacitance absorbs charge, which is subsequently discharged on the falling edge. This leads to a capacitive high frequency current which must be supported by the inverter output. The charging current increases with increasing switching frequency. This can be a particular problem for small drives, for which the cable charging currents may be significant compared with the drive's current rating. If excessive, the cable charging current can result in a drive trip in which the inverter is disabled. As a result, the power ratings set for drives can conventionally be quite restrictive, to avoid tripping or the creation of unsafe operating conditions.
Conventionally, the calculation of the power rating for a drive is based on the cable being a predetermined length. Each drive also has a maximum cable length rating, with potentially unsafe conditions being created if the drive is used with a longer length of cable. The power rating of drives is set according to this predetermined maximum allowable cable length, such that unsafe conditions will not be created when cables of this length or shorter are used. In particular, the temperature of the inverter in the drive will not exceed a predetermined value.
It has been appreciated by the inventor that, with the conventional power rating system as described above, the predetermined power rating (and other ratings for the drive) can be overly restrictive when a motor cable is used having properties different from those of the cable used in determining the ratings of the drive. An example of this situation is when a shorter cable than the maximum cable length of the rating is used.